Polyethylene can be cross-linked in a number of different ways, the modes most commonly used being peroxide cross-linking, radiation cross-linking, or silane cross-linking. In both radiation and peroxide cross-linking, the cross-linking is a radical reaction. As a result of irradiation, or decomposition of peroxides, radicals are formed in the polymer chains. When these radicals are combined, covalent bonds, so called cross-links, are formed between the polymer chains. In silane cross-linking silanol groups react with each other and form cross-links.
In peroxide cross-linking, polyethylene is first blended with peroxides and with additives, whereupon the mixture is extruded, for example, to make cables or pipes. The cross-linking takes place in the molten state at about 300.C, generally under nitrogen atmosphere after extrusion at higher temperature in a so-called vulcanizing line. The peroxides are decomposed at high temperature and form radicals, which produce radicals in the polymer chains. When polymer radicals are combined, a covalent bond is formed between two molecules. After a sufficiently high number of bonds, the polymer becomes insoluble. The degree of cross-linkage is usually measured by means of solution extraction in accordance with ATSM D2765 standard.
Radiation cross-linking takes place at room temperature after extrusion, as a rule in a separate step. By means of radiation, only amorphous parts of a polymer can be cross-linked. Radiation has very little effect on crystalline parts of a polymer. Thus, a low-density polyethylene is easier to cross-link than a high density polyethylene. As a source of radiation, electron accelerators or a source of gamma radiation are usually used. The energy of an electron accelerator suitable for cross-linking applications varies in the range of 500 keV to 10 MeV, and the output in the range of 10-20 KW. Electron radiation has a lower depth of penetration than does gamma radiation with the same radiation energy. It is for this reason that gamma radiation is used for radiation cross-linking of large pieces. Gamma radiation is derived from radioactive isotopes, for example .sup.50 Co or .sup.137 Cs, or it can be produced by means of an electron accelerator by directing the electron beam, e.g. at a tugsten target, whereby bremsstrahlung is produced. In both cases, the source of gamma radiation has a considerably lower absorbed dose rate than does an electron accelerator. Cross-linking of cables by means of an electron accelerator usually takes only a few seconds.
Polyethylene can be grafted with vinylsilanes, and from this graft copolymer a product can be prepared immediately or later on. Afterwards, the product can be cross-linked with water vapor (monosil and Sioplas techniques). The above technique is also employed for cross-linking of ethylene-vinylsilane copolymers. In order to enhance the cross-linking, a catalyst is usually used. For example, tin dibutyl-dilaurate. Silane cross-linking is slow, the reaction speed depends on the diffusion of the water vapor in the polymer. Thus, silane cross-linking is best suitable for products which have a thin wall, for example low-voltage cables.
The resistance to heat and the maximum use temperature of polyethylene can be increased by cross-linking. At high temperatures, for example above the crystalline melting point of polyethylene, a polyethylene that has not been cross-linked flows, whereas a cross-linked polymer retains its shape because the polymer chains cannot move in relation to each other. Thus, a cross-linked polyethylene object retains its shape better than an object that has not been cross-linked. Irrespective of the method of cross-linking, the mechanical strength of a polymer at high temperature is improved.
In other applications, cross-linking is necessary in order to be able to use polyethylene, LDPE (low-density polyethylene) can be used constantly at a maximum temperature of 70.degree. C. and momentarily at 90.degree. C. Cross-linked polyethylene can be used constantly at 90.degree. C. and momentarily even at 250.degree. C. Other polyethylene types, such as HDPE (high-density polyethylene) withstand heat better than LDPE, because of higher melting point (density and crystalline). Cross-linking permits the use of polyethylene within a wide temperature range, which is important in many applications, such as cable or hot water piping applications. In cross-linking of insulation layers for cables and hot water pipes it is very important that a sufficiently high degree of cross-linking is achieved in order to achieve a sufficient resistance to heat. Measured by means of solution extraction in accordance with the ASTM D 2765 standard, as a rule, a degree of cross-linking higher than 70% is required for cable and pipe applications. (Roberts B. E., and S. Verne, Plastics and Rubber Processing and Applications 4 (1984), pp 135-139).
Cross-linked polyethylene can also be used for shrink applications. The polymer is extruded and cross-linked after which the product is stretched and cooled. The product cools in the stretched form, and on heating the product is restored to the same shape that it had before stretching. In this way, it is possible to prepare shrinkwrap film, pipes and joints. The shrinkage of a material that has not been cross-linked depends mainly on the orientation obtained by the polymer during extrusion. Cross-linking permits easy control of the shrinking quality; moreover, the shrinking force of a cross-linked polymer is high as compared with a polymer not cross-linked. Shrink applications, such as shrinkwrap film, shrink bag or different types of joints, usually require a lower degree of cross-linking, as a rule about 30-50%.
Cross-linked polyethylene can also be utilized in the production of foam plastic. The polyethylene, the foaming agent, and the additives are extruded and cross-linked. The cross-linked polymer is foamed, whereby the polymer cools to the stretched form. An advantage of cross-linked polyethylene foam is smaller and more uniform cell size and, consequently, improved mechanical properties.
If a polymer has a high molar mass, an increase is obtained in the mechanical properties of the polymer, such as its tenacity, tensile strength, and resistance to heat. A polymer with a higher molar mass can be cross-linked more readily, because fewer cross-links are required to make the polymer insoluble. Thus, a polyethylene that can be cross-linked readily has a higher molar mass. On the other hand, the quality of working of a polymer deteriorates decisively when the molar mass of the polymeris increased. The melt index of polyethylene is measured by means of the method of the ASTM D-1238 standard at 190.degree. C. The melt index gives a picture of the fluidity, and consequently also of the workability and of the molar mass of the polymer.
In the cross-linking, a gel dose means a radiation dose that is required to form one cross-link per molecule. With this radiation dose the polymer becomes insoluble since all the polymer chains are linked with each other. In practice, the cross-linking takes place in a random way, thus, first a part of the polymer is not dissolved and, when the radiation dose becomes larger, the gel concentration increases (Bradley R., Radiation Technology Handbook, Marcel Dekker Inc. 1984).
The polyethylene copolymers ethene-vinylacetate (EVA), ethylene-buylacrylate (EBA), ethylene-methylacrylate (EMA), or ethylene-ethylacrylate-copolymer (EEA) are softer and more elastic materials than polyethylene. Thus, being not cross-linked, these copolymers also have an inferior resistance to heat, as compared with polyethylene. Depending on the amount of comonomers, the copolymers are more amorphous than polyethylene. As a rule, the concentration of comonomers is about 1-30% by weight. Owing to their lower crystallinity, copolymers are cross-linked about 5-15% better than LDPE (polyethylene). Owing to their lower melting point and softening point, copolymers must, as a rule, be cross-linked to a higher extent than polyethylene in order to achieve the same mechanical strength as LDPE (polyethylene) at a temperature of 100.degree.-130.degree. C. At higher temperatures (&gt;150.degree. C.), the mechanical strength depends mainly on the degree of cross-linking. A low-density polyethylene whose melt index is 5-10 g/10 min, measured with a weight of 2.16 kg, requires a radiation dose of about 200-300 kGy to reach a cross-linking degree of 60%. LDPE requires approximately the same dose. However, HDPE requires a higher dose, 250-350 kGy. EVA, EBA and other acrylate copolymers are cross-linked a little better and reach a cross-linking degree of 60% with a dose of about 150-250 kGy. These values are guide values for qualities that have a melt index of 3-10 g/10 min.
Ethylene-vinylalcohol-copolymer is very difficult to cross-link with peroxides or by electron radiation.
By copolymerization of ethylene and diens, it is possible to prepare polymers that contain double bonds. Many polyethylene qualities and ethylene-propylene rubbers that can be cross-linked are copolymers or terpolymers of ethylene and a diene. These polymers are usually prepared by means of coordination polymerization. In many cases, 1,4-hexadiene has been used as a comonomer. Other dienes that have been commonly used as a comonomer are, for example, 5-methyl-1-4,hexadiene, 4-methyl-1,4-hexadiene, 1,6-octadiene, cyclohexadiene, dicyclopentadiene, or 5-ethylidene-2-norbornene (JP 59106946-A, JP 57098534-A, JP 570599333-A).
Polyethylene copolymers that contain double bonds are cross-linked with peroxides as much as 50% better than saturated polyethylenes prepared by means of a corresponding catalyst. By radiation, these unsaturated polymer qualities are cross-linked only slightly better than corresponding saturated polymer qualities.
It is well known that an acrylate double bond reacts particularly sensitively to radiation or peroxide. Most lacquers and paints hardening by means of ultraviolet light (UV) or electron radiation are based on acrylated epoxy, urethane or polyester oligomers. As a rule, these oligomers contain 3 to 50 acrylate double bonds. To harden these oligomers, a radiation dose of 10-30 kGy is enough for substantially complete polymerization. Oligomers have a relatively high viscosity, for which reason the paints and lacquers are mixtures of oligomers and monomers. The monomers that are used most commonly are hexane-diol-diacrylate (HDDA), tripropyleneglycol-diacrylate (TPGDA), trimethylol-propanetriacrylate (TMPTA), or n-vinylpyrrolidone (NVP) (Holman R., YV & EB Curing Formulations For Printing Inks, Coatings & Paints, SITA Technology 1984).
It is known in the prior art that it is possible to improve the cross-linking quality of polymers by blending them with multi-functional acrylate monomers. In such case, 1-10% by weight of mono-, di- or tri-functional acrylate monomers of allyl monomers have been used, such as tetraethyleneglycol-diacrylate (TEGDMA), trimethylolpropane-trimethacrylate (TMPTMA), or triallylcyanurate (TAC), note, DE 1544804-B.
Polyethyleneacrylate monomer mixtures, however, involve the problem that it is difficult to disperse a polar monomer in a non-polar matrix plastic. Acrylate and allyl monomers that are blended in polyethylene tend to be carried out of the matrix plastic and to gather on the surface, causing so-called sweating. Owing to the sweating, the object must be cross-linked as soon as possible after extrusion. In the stage of cross-linking, the monomers are polymerized as a homopolymer of the acrylate of allyl monomers concerned. Thus, monomers that are capable of forming bonds between the different polymer chains in the matrix plastic are present only at the boundary face between the monomer phase and the matrix plastic.